I’ll second Carl Hiaasen, Donald Westlake and PJ O’Rourke.
As S Ujest above shows, O’Rourke has the best chapter names. My favourite: “How to Drive Fast on Drugs While Getting Your Wing-Wang Squeezed”.
I’m definitely going to check out a few others mentioned.
Some more classics, as yet unmentioned: Anita Loos’ Gentlemen Prefer Blondes and its sequel, Gentlemen Marry Brunettes; and Patrick Dennis’ wickedly hilarious **Little Me. **
Feel this Book:D
For funny and quick, I highly recommend e by Matt Beaumont. It’s a novel consisting entirely of e-mails sent within a fictitious ad agency in London – a very amusing no-brainer of a book.
As counterpoint, I have to mention Ernest Vincent Wright’s novel Gadsby, in which the letter “e” is not used even once.
Big Trouble by Dave Barry.
A second for Big Trouble by Dave Barry.
A couple of Colin McEnroe books are just hilarious. The best is, “How To Lose Weight Through Great Sex With Celebrities (The Elvis Way)”. Another is “Swimming Chickens”. Silly, hilarious stuff.
I haven’t seen Fran Lebowitz’s name mentioned, so I’ll remedy that now.
In honour of Canada Day:
I’d recommend the first three volumes of “The Bandy Papers” by Donald Jack. Three Cheers for Me, That’s Me in the Middle, and It’s Me Again are the first trilogy, which IMHO were the best. Some of the later books are good (e.g. Me Bandy, You Cissie) but some are much weaker than the early ones.
The books will appeal to people who enjoy flying ace stories like The Dawn Patrol, absurdity of war stories like Catch-22, or any of the English “Ripping Yarns for Boys”/Biggles type books. Some of the humour is broad, some is quite black, but it’s all very funny.
An Underachiever’s Diary by Benjamin Anastas
And, by reputation (I haven’t read it), the supposedly similarly poignant/funny literary precedent that book has been compared to,
A Fan’s Notes by Frederick Exley.
I’ve read about half of Hiassen’s and thoroughly enjoyed them, but if I had to recommend just one, it’d be Double Whammy.
Re. film scripts and teleplays, I downloaded Bruce Robinson’s corrosively funny Withnail & I off the 'net for free. The collected teleplays for the Fawlty Towers series has been out in paperback for several years, as are at least some of the Seinfeld scripts.
“I’m trying to enjoy my holiday…” [Withnail] 
For non-fiction humour you could try Bill Bryson.
I will second The Bandy papers. Laughed so much my gut hurt. Particularly “3 cheers for me” (though they really should be read in sequence).
Also (in honour of all this Lord of the Rings broohaha) Bored of the Rings by the Harvard Lampoon. Silly, but good for a cheap chuckle.
Humour can be a pretty subjective thing. Books I found funny include:
For lower-key humour, maybe Tom Wolfe (The Electric Kool-Aid Acid Test), Gita Mehta (Karma Cola), anything by James Heriott, compilations of the Fortean Times (Strange Days) or the first Letters from a Nut (Jerry Seinfeld writing as Ted Nancy).